1. Field of Invention
This invention relates to a method for preparing catalysts useful in the manufacture of dicarboxylic acid anhydride by the oxidation of hydrocarbons. More particularly, it is directed to the preparation of catalysts suitable for producing maleic anhydride from n-butane, n-butenes, 1,3-butadiene or mixture thereof.
2. Description of the Prior Art
The preparation of oxide catalysts comprising vanadium and phosphorus for use in a vapor phase oxidation of a hydrocarbon feed is known in the art. Various catalysts have been proposed wherein during the catalytic preparatory step, pentavalent vanadium in the catalyst is reduced to a valence state below +5 using a reducing agent. The conventional methods of preparing the catalysts involve combining a vanadium compound, a phosphorus compound, and when specified, promoter element compounds in an acidic reducing medium under conditions which will provide vanadium in a valence state below +5 to form a catalyst precursor, thermally convertible to an oxide or an oxide complex catalyst; recovering the catalyst precursor; and calcining the catalyst precursor at temperature of about 350.degree. C. to about 600.degree. C. for at least two hours. The reducing agents employed are usually solutions of mineral acids, particularly hydrochloric acid and phosphorous acid, or organic reducing agents, especially oxalic acid. For example, U.S. Pat. No. 3,985,775 to Harrison, et al. discloses the oxidation of n-butane using a mixed vanadium-phosphorus catalyst which is prepared by dissolving vanadium pentoxide in concentrated hydrochloric acid and adding 85% phosphoric acid to the resulting solution to yield a phosphorus to vanadium ratio of 0.5:1.0 (preferably 1.2:1.0). The resulting solution is then concentrated by heating to give a 50% solid aqueous slurry and then dried to constant weight at 150.degree. C.; the dihydrate produced is preferably in or converted into, particulate form for processing through the subsequent phase transition. U.S. Pat. No. 3,975,300 to Burress, et al. teaches the use of organic reducing agents, such as glycol, sucrose, ethylene glycol, and propylene glycol, in the preparation of vanadium-phosphorus complex catalysts. U.S. Pat. No. 4,002,650 to Bremer, et al. discloses the oxidation of n-butane using a catalyst of the formula of V.sub.0.5-3 P.sub.0.5-3 U.sub.0.1-0.5 O.sub.x. The preferred preparation of the catalyst involves refluxing a mixture of vanadium pentoxide, concentrated hydrochloric acid, and uranyl acetate. To this mixture is added 85% phosphoric acid. The mixture is evaporated at atmospheric pressure, dried at 110.degree. C. and activated by heating in an air flow at 482.degree. C. for 16 hours. U.S. Pat. No. 3,888,886 to Young, et al. discloses the oxidation of n-butane using a vanadium-phosphorus-oxygen complex catalyst having a phosphorus:vanadium atomic ratio of 0.5:2, promoted or modified with certain transition metals, preferably zirconium, chromium, iron or hafnium. These catalysts are prepared by refluxing a reaction mixture of vanadium oxide, phosphoric acid, a hydrogen halide (usually hydrochloric acid) and a specified promoter metal compound. U.S. Pat. No. 4,018,709 discloses the vapor phase oxidation of 4-carbon n-hydrocarbons using catalysts containing vanadium, phosphorus, uranium or tungsten or a mixture of elements from zinc, chromium, uranium, tungsten, cadmium, nickel, boron and silicon. Preferably, the catalytic complex also contains an alkali metal or an alkaline earth metal, especially lithium, sodium, magnesium or barium, as active components. Catalysts are prepared in a 37%-hydrochloric acid solution. U.S. Pat. No. 3,980,585 to Kerr, et al. discloses the preparation of maleic anhydride from n-4C hydrocarbons in the presence of a catalyst containing vanadium, phosphorus, copper, oxygen, tellurium or a mixture of tellurium and hafnium or uranium. The process may also be conducted in the presence of a catalyst containing vanadium, phosphorus, copper, at least one of Te, Zr, Ni, Ce, W, Pd, Ag, Mn, Cr, Zn, Mo, Re, Sm, La, Hf, Ta, Th, Co, U, Sn and optionally an element from Groups IA or IIA. This patent exemplifies the use of oxalic acid in the preparation. U.S. Pat. No. 4,016,105 teaches the preparation of a V-P complex catalyst in an aqueous phosphoric acid solution using an organic acid or aldehyde and secondary alcohol as reducing agents.
U.S. Pat. No. 3,907,835 to Kobylinski discloses the production of maleic anhydride from benzene, butene, butadiene, butanol-2 or pentanol-2 using a catalyst of the formula U.sub.1-3 O.sub.6-16 P.sub.1-4 H.sub.0-4, and optionally containing vanadium. Where vanadium is present, the catalyst is prepared by mixing an anhydrous uranium salt with vanadyl oxalate (vanadium to uranium is 0.1:1 to 0.1:2) and adding enough concentrated phosphoric acid to give a uranium to phosphorus ratio of 0.2:1 to 2:1 molar ratio and phosphorus to oxygen ratio of 0.1:1 to 0.35:1 to precipitate the catalyst which is dried at 29.degree. C.-140.degree. C. and heated to 420.degree. C.-500.degree. C. The vanadium content of this catalyst is preferably 6 to 40 weight percent. The anhydrous uranium salt is especially a phosphate prepared by addition of a base to an aqueous solution of uranyl nitrate in concentrated phosphoric acid and drying and calcining to obtain catalytic precipitate.
Of particular interest is U.S. Pat. No. 3,977,998 to Freerks, et al., which discloses the oxidation of n-butane in the presence of a phosphorus-vanadium-oxygen complex catalyst, wherein the phosphorus to vanadium atom ratio is 1-2:2-1, the catalyst being prepared by (a) contacting a vanadium compound and a phosphorus compound in acid solution containing a reducing agent under conditions which will provide at least 50 atom percent of vanadium in tetravalent form; (b) separating the prepared catalyst precursor and (c) calcining the catalyst precursor at 350.degree. C.-660.degree. C. for at least two hours, the improvement comprising that the calcination is effected in an inert atmosphere. The reducing agent may be a hydrogen halide acid or oxalic acid, but is preferably a mixture of phosphoric acid containing sufficient phosphorus acid to reduce V.sup.+5. Preferred catalyst precursors have a phosphorus to vanadium atom ratio of 1:1 to 1.5:1, especially 1:1 to 1.2:1. Exemplified in this patent is the use of a catalyst of the formula P.sub.1.05 V.sub.1 O.sub.x prepared by digesting vanadium pentoxide in a mixture of water, 85% phosphoric acid and 99.4% phosphorus acid. This mixture was heated to 100.degree. C. in an autoclave which was then sealed; the mixture was heated for 3 hours at 145.degree. C. and solid catalyst precursor was collected and slurried in 20 weight percent water. Viscous putty was extruded through a 0.35 cm. diameter die and cut into pellets, then air-dried and heated to 125.degree. C. Pellets were heated in a muffle furnace to 350.degree. C. for one hour and then to 375.degree. C. an additional hour. The air in the furnace was replaced by nitrogen gas, and the temperature was raised to 500.degree. C. over a five hour period. The catalyst was cooled rapidly to room temperature under nitrogen gas and possessed a 93 atom percent of vanadium as V.sup.+4.
All of these teachings in the prior art have failed to achieve the desirable results obtained by the use of the present invention. The activity and quality of catalysts prepared using conventional prior art techniques are diverse because catalysts containing vanadium and phosphorus are especially sensitive to their mode of preparation. Using the present invention, reproducible catalysts of enhanced activity and selectivity are obtained.